This invention relates to improved power take off devices (PTO) for use in wave energy conversion systems (WECs).
In general, WECs include: (a) a float (shell) which moves in phase with the waves; (b) a spar or column which is either stationary with respect to the float or moves out of phase relative to the float; and (c) a power take off device (PTO) coupled between the float and spar to convert their relative motion into a useful form of energy (e.g., electric power).
Many different types of PTOs have been suggested. However, there exists a need to have a PTO which is more efficient, more reliable and more economical than those presently known.
Present WEC technology, as shown in FIG. 1, relies on the float 1 moving along and in phase with the wave surface but guided by the spar 2 which has a submerged end connected to the sea bed or to a heave plate 3 which renders the spar relatively stationary. The relative linear motion between the float and spar is transferred via a bridge 4 through a linear thrust rod 5 to drive a power take-off system 10 located in the spar. As the power take-off system is generally placed inside the spar, a water and air tight chamber needs to be formed within the spar and a linear seal 30 at the top of the spar.
A problem with the present technology is that a linear seal system has to be placed at the top of the spar to interface the thrust rod and ensure that water and air will not enter into the spar. The seal system also serves as a linear bearing system to guide the thrust rod. The linear seal is a weak link in the system because it is extremely difficult to provide a reliable seal of this type. Thus, a goal is to eliminate the need for the linear seal system. It is desirable to replace the linear seal with a rotary seal type system which has had much more extensive development and is generally more reliable.
Another problem with current designs is that the thrust rod needs to transfer the relative linear motion between the float and the spar while interfacing with the linear seal. In addition to generally limiting the length of the stroke, the thrust rod has to handle significant loads in both compression and tension and must also have high wear resistance. The linear thrust rod is one of the most expensive and weakest items in current WEC designs. The thrust rod also has limited scalability in larger systems. It is therefore desirable to replace the thrust rod with a more reliable and economical system
The problems with the thrust rod and linear seal are avoided in systems embodying the invention. In contrast to the prior art shown in FIG. 1, in WEC systems embodying the invention the transfer of float motion via a thrust rod is eliminated as well as the need for a linear seal.